Coal is one of the least expensive and most abundant sources of energy in the United States. The major end use of this energy source is in the conversion thereof to produce electricity. The economic conversion of coal to electricity has been the subject of a great deal of research and study. For example, coal and other combustible materials have for many years been burned in a wide assortment of furnaces, and the heat of combustion has been used to produce steam for the powering of turbogenerators. More recently, there has been substantial growth of atmospheric fluidized-bed combustors (AFBC) and entrained fuel-type combustors for this purpose. The addition of lime or limestone to the combustors has enabled simultaneous desulfurization of the flue gases produced during the combustion process. This treatment has been particularly effective in fluidized-bed combustors, often achieving more the 90 percent removal of the sulfur entering with the coal.
A further development of the AFBC is the pressurized fluidized-bed combustor (PFBC) which provides a new approach to the generation of power. In this approach, the combustor is pressurized and the hot flue gases are expanded through a gas turbine for generating electrical power. The gas turbine is a device which generates power by the expansion of gases, usually combustion product gases. The combustor producing the gases is often a integral part of the turbine, although it can be physically separated. A gas turbine with an integral coal burning combustor can be considered as providing what is essentially a one-step process for converting coal to electricity and thus represents an exceptionally simple approach to coal conversion.
In addition to the pressurized combustor, a conventional PFBC typically includes coils located in the combustor which carry water that is converted to steam. The steam can be utilized in a steam-injected gas turbine or in a separate steam turbine. In order to limit the formation of nitrogen oxides and to efficiently remove sulfur oxides, the temperature in the combustor must be maintained near 1,600.degree. F. This has been done in current designs by heat removal through the generation of steam in the internal coils. A discussion of the background and current state of development of PFBC's for power production may be found in "Proceedings: Pressurized Fluidized-Bed Combustion Power Plants," EPRI CS-4028, May 1985.
The pressurized fluidized-bed combustor is still under development and is not considered to be fully ready for commercialization because of uncertainty as to the economics of this approach. One of the items contributing to this uncertainty is the use of internal coils referred to above in providing a means of temperature control to the pressurized fluidized-bed combustor system in that such an approach adds significantly to the costs of the system.